1. Field of the Invention
The present invention relates to an optical illumination apparatus illuminating image forming means with light from a light source, and an image projection apparatus illuminating an image, which is formed in the image forming means, with illumination light, and projecting the image on a screen with enlarging the image with a projection lens.
2. Description of the Related Art
In order to obtain an image on a large screen, an image projection apparatus is used, the image projection apparatus which illuminates small image forming means, forming an optical image according to a picture signal with light from light source, and projects the optical image on a screen with enlarging the optical image with a projection lens. As the image forming means, transmissive liquid crystal panels are widely used in practical use, the transmissive liquid crystal panels each of which is in an active matrix method, has such configuration that polarizing plates are arranged in crossed Nicols in both sides of each twisted nematic liquid crystal cell, and modulates light with using polarization. Two lens array plates configured by a plurality of lenses are used in an optical illumination apparatus illuminating a liquid crystal panel with light from light source (for example, U.S. Pat. No. 5,098,184). The two lens array plates efficiently and uniformly illuminates the liquid crystal panel by dividing a light beam incident on one of the lens array plates, which is located in a light source side, into multiple light beams, and superimposing each light beam on the liquid crystal panel.
In addition, as an optical illumination apparatus for an image projection apparatus using an liquid crystal panel using polarization, an optical illumination apparatus is disclosed, the optical illumination apparatus which constructs a polarization transforming optical member transforming natural light into light in one polarization direction with using a polarization separating prism which is polarization separating means and a half-wave plate which is polarization rotating means, increases an efficiency of light utilization in the image projection apparatus, and makes the image projection apparatus brighter (for example, U.S. Pat. No. 5,098,184). Furthermore, so as to make an image projection apparatus brighter, an optical illumination apparatus using a plurality of light sources is disclosed (for example, Japanese Patent Laid-Open Nos. 6-265887 and 6242397).
FIG. 14(a) shows an image projection apparatus introducing a conventional optical illumination apparatus using a plurality of light sources. Light emitted from two discharge lamps 1 and 2, which are light sources, is converted into nearly parallel light beams by being converged by respective concave mirrors 3 and 4. Respective parallel light beams enter into a first lens array plate corresponding to them. The first lens array plate 5 is configured by a plurality of rectangular lenses, divides the incident light beams into multiple light beams with respective rectangular lenses, and converges the multiple light beams on a plurality of respective lenses in the second lens array plate 6. Multiple minute light source images are formed on the respective lenses in the second lens array plate 6. The second lens array plate 6 focuses and superimposes images of the respective lenses of the first lens array plate 5 on liquid crystal panels 16 to 18.
The light outgoing from an optical illumination apparatus 7 illuminates liquid crystal panels 16 to 18 corresponding to respective rays of colored light after being divided with dichroic mirrors 8 and 9 into three primary colors: green; red; and blue. In this manner, the optical illumination apparatus 7 performs uniform illumination by superimposing multiple light beams, which are divided, on the liquid crystal panels. Relay lenses 11 and 12 correct the difference between intensities of respective illumination light to the liquid crystal panels that are caused by different optical paths of the illumination light that are distances from the second lens array plate and liquid crystal panels. Field lenses 13 to 15 converge the illumination light to the liquid crystal panels 16 to 18 on a pupil surface 21 of a projection lens 20. After the three primary colors, which are blue, green, and red, outgoing from the liquid crystal panels 16 to 18, are synthesized with a dichroic prism 19, the three primary colors synthesized enter into the projection lens 20. The projection lens 20 enlarges and projects images of liquid crystal panels 16 to 18 on a screen (not shown). Since the plurality of light sources are used, it is possible to configure a bright image projection apparatus.
FIG. 14(b) shows a mode of light source images formed on the pupil surface 21 of the projection lens 20. The two light sources 1 and 2 are made to be minute light source images 24 with the lens array plate, and light source image groups 22 and 23 are formed.
Generally speaking, so as to increase the brightness of an image projection apparatus, it is sufficient to increase the power consumption of a discharge lamp. Nevertheless, there is such a task that, if the power consumption is increased with keeping the life of the discharge lamp, a light emitting portion becomes large and hence an efficiency of light utilization decreases. For this reason, it is possible to more efficiently increase the brightness of the image projection apparatus by using a plurality of light sources whose power consumption is relatively small. In the configuration of a conventional optical illumination apparatus using a plurality of light sources like the apparatus shown in FIG. 14(a), two light sources are symmetrically located with sandwiching an optical axis of a projection lens. In such a case, images of light sources formed on the pupil surface of the projection lens are formed as the images from two light sources with sandwiching the optical axis as shown by the pupil surface of the projection lens in FIG. 14(b). Since there is a vignetting in a projection lens, peripheral illumination is lower than central illumination on a screen. This is because an eclipse arises in the light source images on the pupil surface of the projection lens due to the vignetting. Therefore, if luminescence properties of the two light sources located with sandwiching the optical axis are different from each other, light source images contributing to the brightness in the peripheral part of the screen are different from each other. Hence, irregular color in a projected image arises on the screen. In addition, there arises such a task that, if one of two light sources is burnt out, a luminance distribution on the screen becomes uneven.
Furthermore, if such an optical illumination apparatus is introduced in such an image projection apparatus that is shown in FIG. 14(a), in regard to red light among the three primary color light, light source images formed on the pupil surface of the projection lens are inverted against the optical axis. Therefore, as for respective light source images on a pupil surface of a projection lens, green and blue images of the light source 1 are formed in an area 22, and red images of the light source 1 are formed in an area 23. Moreover, green and blue images of the light source 2 are formed in an area 23, and red images of the light source 2 are formed in an area 22. For this reason, even if luminescence properties of the two light sources are slightly different from each other, a mode of eclipses in the light source images changes due to vignetting in the projection lens. In consequence, there arises such a task that large-scale irregular color on the screen arises.
Therefore, in case an optical illumination apparatus and an image projection apparatus are configured by a plurality of light sources being used, it is necessary to configure the optical illumination apparatus wherein light source images on a pupil surface of a projection lens that are formed by respective light sources are as symmetrical as possible against an optical axis, and wherein the optical illumination apparatus is highly efficient.
Furthermore, in the configuration shown in FIG. 14(a), a small F-number of the projection lens is required so as to efficiently introduce light from the optical illumination apparatus. Nevertheless, there is such a task that the decrease of the F-number of the projection lens leads to the increase of size and cost of the projection lens.
In addition, there is also such a task that first and second lens array plates are required in connection with two concave mirrors and hence manufacturing cost increases.